You can put this solution on YOUR website! Let M = Mary's age
then because "Susan is six years older than Mary."
M+6 = Susan's age
.
From:"She is twicw as old as Mary"
M+6 = 2M
Solve for M by subtracting M from both sides:
M+6-M = 2M-M
6 years old = M (Mary's age)
.
Susan's age:
M+6 = 6+6 = 12 years old (Susan's age)